As the environmental, health and industrial impact of pollutants increases, it is becoming increasingly important to develop new methods for the rapid and efficient removal of a wide range of contaminants from polluted waters and other liquids. The invention is directed to a high efficiency method for the remediation of large quantities of liquid, operating at low to moderate ambient pressures, in order to reduce environmental or health risks or to purify the water for use in industrial processes. Moreover, this method reduces or eliminates the use of chemical additives. Rather, decontamination is achieved through the use of submerged liquid jets which trigger hydrodynamic cavitation events in the liquid. These cavitation events drive chemical reactions, by generating strong oxidants and reductants, efficiently decomposing and destroying contaminating organic compounds, as well as some inorganics. These same cavitation events both physically disrupt or rupture the cell walls or outer membranes of microorganisms (such as E. coli and salmonella) and larvae (such as Zebra mussel larvae), and also generate bactericidal compounds, such as peroxides, hydroxyl radicals, etc., which assist in the destruction of these organisms. Following disruption of the cell wall or outer membrane, the inner cellular components are susceptible to oxidation.
There are many means for removing contaminants and inclusions from liquids, including filtration, stripping, adsorption, absorption, and ion exchange. One technique employs oxidation of contaminants, in which chemical reactions are induced with oxidization agents to break the compounds down into simpler substances which, in turn, may also be oxidized. In the case of organic contaminants, the ultimate end products of oxidation reactions are typically nontoxic substances such as water and carbon dioxide. Thus, oxidation may completely destroy the contaminating substances, rather than merely removing them from the water for disposal elsewhere.
Oxidation reactions may be induced by a variety of means, such as the use of various chemicals, ozone, or supercritical water, or photochemical oxidation where ultraviolet radiation is used to produce hydroxyl radicals, which are strong oxidizing agents. These methods are often costly. Oxidation reactions also can be initiated by inducing hydrodynamic cavitation events in the solution, that is, by inducing the growth and rapid collapse of cavitation bubbles (also called cavities, microcavities or microbubbles) in the liquid. According to one theory, the generation of a "hot spot" (a local high temperature and pressure region) upon cavity collapse is responsible for dissociating the water molecules in aqueous liquids to produce hydroxyl radicals. Other oxidizing radicals may be formed in aqueous solutions as well as in non-aqueous environments. Oxidation reactions thus occur at the site of the collapsing cavity or bubble.
Systems using ultrasonically-induced cavitation have been found to promote a wide range of physical and chemical reactions and to be capable of at least partially oxidizing dilute aqueous mixtures of organic compounds. This may be achieved using ultrasonic horns to send a high intensity acoustic beam into the solution and excite microcavities. U.S. Pat. No. 4,076,617 (Bybel et al.) utilizes cavitation induced by acoustic means to create an emulsion of the waste material in water followed by application of ozone to oxidize the emulsified waste. U.S. Pat. No. 5,198,122 (Koszalka et al.) teaches the application of ultrasonic energy to contaminated liquids in the presence of oxidants. However, the efficiency of such ultrasonic devices is limited by achieving cavitation in the form of a cloud of cavitation bubbles only in a relatively small region near the surface of the ultrasonic source. Moreover, the efficiency of transfer of electric power into ultrasonic energy and then into the liquid itself is quite low, of the order of about 15%.
Other methods employ venturi flow to induce cavitation in contaminated aqueous solutions by relying on the pressure drop and subsequent pressure rise associated with flow through the venturi to cause cavitation bubble nuclei to grow and collapse. However, these methods are limited by their complexity and efficiency, and may require additional treatments, such as with chemical oxidizing agents, ultraviolet radiation, or both, to achieve the desired water purity. U.S. Pat. No. 4,906,387 (Pisani) and U.S. Pat. No. 4,990,260 (Pisani) teach first inducing cavitation in contaminated water which has been treated to provide hydroxyl free radicals and then irradiating the cavitated treated water with ultraviolet radiation. Cavitation is induced by passing the water through a cavitation critical flow constriction, shown in the figures to be a venturi-type constriction (that is, a cylindrical conduit of gradually decreasing and then gradually increasing inner diameter).
U.S. Pat. No. 5,326,468, U.S. Pat. No. 5,393,417, and U.S. Pat. No. 5,494,585 (the Cox patents) teach the production of oxidation by action of a cavitation venturi which is operated with a throat size and pressure drop to incur cavitation in the water. The Cox cavitation venturi comprises an inlet passage which converges in a cone, and a variable throat which is controlled by feedback from various sensors. The cavitation phenomenon which results in the formation and collapse of micro-bubbles is said to be contained in the expanding diameter outlet body of the cavitation venturi, the large end of which is essentially the same diameter as the inlet passage to the venturi. Sensors and programmable control feedback are used to adjust the throat of the venturi nozzle to optimize cavitation conditions. Oxidation is continued by the use of high energy ultraviolet radiation and/or hydrogen peroxide injection.
The cavitation taught by Cox requires high velocities and energy in order for cavitation to occur as a result of the pressure drop generated in the liquid. By contrast, the nozzles which create the fluid jets utilized in the present invention are designed to actively intensify, or energize, the cavitation generated by the pressure drop by very rapidly decreasing the inner diameter of the nozzle. This sharply changes the direction of the liquid flow, from flow along the inner periphery of the nozzle to flow toward the centerline of the nozzle and creates a first shear layer, or shear zone, inside the nozzle. This shear results in very rapid local pressure drop regions, thereby intensifying the cavitation events. In a more preferred embodiment, additional interaction with the inner surface of the nozzle can facilitate the formation of large vortical structures. Still further intensification can be obtained in certain embodiments of the invention by submerging the jet nozzles so that a large region is created in the fluid in the vicinity of the nozzle exit where strong shear and resulting low pressures are generated in a second shear layer separating the high speed liquid being ejected from the nozzle from the relatively quiescent liquid into which the jets are discharged. Fluid jet cavitation thus is considerably more aggressive than cavitation generated in a venturi-type constriction nozzle, in which the fluid flow continues to adhere to the inner wall of the conduit throughout the length of the venturi.
Submerged jet nozzles have been used to generate a highly concentrated and focused stream of cavitation in various fluids for the purpose of mechanically eroding, cutting, cleaning, or drilling into solid surfaces. See, for example, U.S. Pat. No. 4,508,577 (Conn et al.), U.S. Pat. No. 4,262,757 (Johnson et al.), U.S. Pat. No. 4,389,071 (Johnson et al.), U.S. Pat. No. 4,474,251 (Johnson et al.), U.S. Pat. No. 5,086,974 (Henshaw) and U.S. Pat. No. 4,681,264 (Johnson et al.) which describe various fluid jets and their use for cleaning, cutting, and the like.
By harnessing the more energetic cavitation induced by a jet nozzle, it has now been found that large volumes of liquid can be more aggressively decontaminated, resulting in increased purity without requiring additional oxidation steps. In a preferred embodiment, the jet nozzle is designed to rapidly raise the local velocity of the fluid in the jet shear layer as close to the nozzle exit as possible and through creation of vortices in the exit flow having high pressure reductions or vapor cavities at their center. In this embodiment, the fluid jet is submerged, so that it is caused to flow through a relatively stationary body of liquid, such as spent fluid from the jets, and vortices are thereby created in a second shear zone between the exiting fluid jet and the surrounding (relatively quiescent) liquid. Low pressures are created in the center of the vortices, which promote the growth and collapse of the vapor cavities, further enhancing the decontamination by cavitation.